1. Field of Art
This invention relates to surface acoustic wave signal processing, and more particularly to a surface acoustic wave signal processor having individual FET taps, separately programmable to provide product mixing beneath each tap in which the mixer efficiency is controllable in amplitude and in phase.
2. Description of the Prior Art
Surface acoustic wave (SAW) signal processing is well known, and may be employed to perform a variety of signal combining/comparing functions, some of which are described in Reeder and Gilden U.S. Pat. No. 4,016,514. These include correlation, convolution, time inversion, and the like. When the SAW signal processors include programmability of the taps, to provide a phase and amplitude programmable, general transverse filter, as in Reeder and Grudkowski U.S. Pat. No. 4,024,480. additional functions, such as programmable correlation, multiplexing and the like may be performed. Programmable taps may be used in conjunction with other SAW device parameters to provide still additional functions, such as discrete Fourier transformation, as disclosed in Reeder U.S. Pat. No. 4,114,116.
The problem with the apparatus described hereinbefore is that a significant amount of per-tap hardware (such as output diode pair structures) must be associated and interconnected with the SAW structure. This is because the operational characteristics employ nonlinear product mixing which is achieved in external devices, and the programming thereof is generated in and applied to external circuitry as well. In the aforementioned devices, the SAW structure itself serves merely to linearly mix the signals with each other, and as such only provides the transversal relationship involved in the process. In order to reduce size, cost and weight, as well as spurious effects in signal conduction, it is desirable to provide SAW signal processors in a more integrated fashion, successful monolithic structures being, of course, ideal.
In the past, attempts have been made to provide direct biphase control in SAW signal processing. For instance, the oldest form of phase programming has been the simple selection of interdigital tap fingers by external circuitry, as described in: Hunsinger, B. J., et al, Programmable Surface-Wave Tapped Delay Line, IEEE Transactions on Sonics and Ultrasonics, Vol. SU-18, No. 3, July 1971, pp. 152-154; and in Moore et al U.S. Pat. No. 3,942,135.
Subsequently, field effect transistors (FETs) were turned to as having potential for more control in surface acoustic wave devices. Examples are described by Claiborne, L. P., et al, MOSFET Ultrasonic Surface-Wave Detectors For Programmable Matched Filters, Applied Physics Letters, Vol. 19, No. 3, Aug. 1, 1971, pp 58-60, by Hickernell, F., et al, An Integrated ZnO/Si-MOSFET Programmable Matched Filter, IEEE 1975 Ultrasonics Symposium Proceedings, pp 223-226, and by Hickernell, F. S., et al, Design and Performance of a ZnO/Si-MOSFET Monolithic Quadriphase Programmable Correlator, 1973 IEEE Ultrasonics Symposium Proceedings, pp 324-327. However, each of these cases is confined to use of amplitude control, generally by means of gate bias, to completely transfer each FET tap between the on and off states, for the purpose of selecting the taps physically located at a correct phase point for phase programming. In early devices of this type, emphasis was placed on using a semiconductor substrate, such as silicon, in order to facilitate fabrication of electronic devices on the substrate for circuit integration enhancement.
More recently, the utilization of a semi-insulating gallium arsenide substrate having a semiconducting epitaxial layer for the formation of FETs directly on a piezoelectric SAW device, has been investigated in a variety of ways. Examples are presented in Staples, E. J., et al, A Review of Device Technology For Programmable Surface-Wave Filters, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-21, No. 4, April 1973, pp 279-287, in Bruun, M., et al, Field Effect Transistors on Epitaxial GaAs as Transducers for Acoustic Surface Waves, Applied Physics Letters, Vol. 18, No. 4, Feb. 1971, pp 118-120, and in Bruun, M., Electronic Properties of Gallium-Arsenide Field-Effect-Transistor Structure Used as Detector for Waves, Electronics Letters, Vol. 8, No. 8, April 1972, pp 215, 216. In the devices reported therein, amplitude control is, of course, possible but phase is selectable only by on/off control of FET taps at selected phase points on the substrate surface.
The use of internal, nonlinear product mixing, as a mechanism for providing versatility in SAW signal processors has also been known. In Davis, K. L., Zinc Oxide-On-Silicon Programmable Tapped Correlator, IEEE Ultrasonic Symposium Proceedings, pp 456-458, there is disclosed a SAW processor employing interdigital electrode taps, each phase-half of which is separately biasable with respect to a grounded silicon substrate to control mixing efficiency amplitude with respect to such phase-half, which in turn designates the phase of the mixer product, due to reversal of the roles of the interdigital tap finger elements from ground/signal to signal/ground. In this sense, the mixing device reported by Davis is programmable only in the same fashion as the earliest tap element switching devices (which did not respond to the results of nonlinear product mixing, but simply wave addition in the substrate).
A FET GaAs Convolver utilizing non-programmed mixing is described briefly in Spierman, A. O. W., Acoustic-Surface-Wave Convolver on Epitaxial Gallium Arsenide, Electronics Letters, Vol. 11, Nos. 25/26, Dec. 1975, pp 614, 615.
Despite the plethora of suggestions for improved devices, and particularly for programmable devices which may be implemented using integrated circuit techniques for nearly-monolithic strutures, there has been an equal dirth of success therein.